An unexplained paradoxical clinical observation is that opioids can produce unexpected abnormal pain (i.e., hyperesthesias including hyperalgesia and allodynia) during their use for pain relief. Such opioid-induced abnormal "pain" (i.e., increased sensitivity to normally non-noxious mechanical and to noxious thermal stimuli) is also readily observed in preclinical models evaluating the effects of sustained systemic or spinal opioid exposure on sensory thresholds. Critically, manipulations which block opioid-induced pain also block the behavioral manifestation of opioid antinociceptive tolerance. Related to this idea is the observation that almost all of the substances reported to block experimental opioid antinociceptive tolerance (e.g., MK-801, CGRP antagonists) block endogenous mediators (i.e., glutamate, CGRP) which promote pain. The mechanisms by which opioids produce pain are not known. Preliminary data show that following sustained exposure to morphine, capsaicin-evoked release of CGRP from spinal tissues is enhanced. This observation may provide a physiological basis for increased excitation to the spinal cord, and the opioid-induced pain which may underlie the behavioral manifestation of opioid antinociceptive tolerance. Our goals are to understand the mechanisms by which spinal opioids produce pain and antinociceptive tolerance. We hypothesize that opioid-induced pain, and antinociceptive tolerance, results from enhanced evoked transmitter release from primary afferents. One local mechanism for such enhanced release may be the result of down-regulation of opioid receptors on primary afferent fibers. Enhanced evoked transmitter release, and pain, will result as a consequence of loss of basal inhibitory tone on the primary afferents. In support of this idea are our preliminary data which show that spinal infusion of DAMGO or "knock-down" of spinal mu opioid receptors (MOR) with antisense produces pain as well as a right shift in the antinociceptive dose-response curve (i.e., antinociceptive tolerance for DAMGO or an analogous right shift in the dose-response curve following antisense knock-down of receptors). Our Aims will evaluate the requirement for receptor down-regulation as a local mechanism of opioid-induced pain by determining the time-course of such pain in animals, receptor down-regulation and capsaicin-evoked CGRP release in spinal cord tissues or DRG cells in culture. These studies will be extended to spinal opioid antinociceptive and cellular (signal transduction) tolerance. Critically, treatments will focus on the use of spinal DAMGO and MOR receptor antisense oligodeoxynucleotides (ODNs), which produce receptor down-regulation, and morphine which does not produce receptor down-regulation in vivo. Blockade of MOR down-regulation will be evaluated to determine if this also blocks pioid-induced pain, enhanced evoked CGRP release and opioid tolerance. Data from these studies will provide new information which will bring together cellular and systems observations to identify a plausible mechanism which may underlie the decrease in spinal antinociceptive potency seen with sustained opioid administration. Clearly, spinal antinociceptive tolerance will also impact the systemic potency of opioids due to the need for supraspinal/spinal synergy. Such insights may be relevant to the use ofopioids for chronic pain.